This proposal describes a molecular approach to the study of the activation and expression of cytolytic effector function by human cytolytic T lymphocytes (Tc) and natural killer (NK) cells. Because of the important physiologic role of these cells in anti-viral and anti-tumor responses, a molecular understanding of the mechanism of the cytolytic reactions mediated by these cells is clearly desirable. The proposed studies focus on the role of the T cell surface protein T11 (CD-2, LFA-2, p50) in cytolysis. Originally identified as sheep erythrocyte binding glycoprotein on human T cells, T11 appears to play an important role in the initial stages of T cell activation. This proposal is based on preliminary studies demonstrating that monoclonal antibodies to particular epitopes on T11 can induce antigen-independent activation of cytolytic effector function in Tc clones. To elucidate the role of T11 in the activation of cytolytic effector function, mutagenesis and selection protocols will be used to isolate T11- variants of functionally active Tc clones. These variants will be used to explore the role of T11 in Ca++ flux and phosphatidyl inositol metabolism during T cell activation. In addition, biochemical and ultrastructural techniques will be used to determine how T cell activation via the T3 antigen-receptor complex affects the expression and distribution of T11. A second part of the proposal deals with the role of T11 in the activation of NK cells. Because anti-T11 antibodies also activate T11+ NK cells to lyse NK resistant targets, the possible role of T11 as a recognition structure on NK cells will be examined by the biochemical analysis of clonal variability in T11 on NK clones. Finally, the ability of anti-T11 antibodies to induce antigen-independent activation of Tc effector function will be exploited in a novel strategy for preparing monoclonal antibodies to lytic effector molecules or lysins released at the site of Tc-target cell interaction. These antibodies will enable a molecular analysis of the lethal hit phase of lymphocyte-mediated cytolysis.